Olefin polymerizations such as ethylene polymerization are frequently carried out using monomer, diluent and catalyst and optionally co-monomers in a loop reactor. The polymerization is usually performed under slurry conditions, wherein the product consists usually of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent, the product being often taken off by means of settling legs which operate on a batch principle to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product is further either transferred to another reactor or discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, additives can be added and finally the polymer is extruded and pelletized. This technique has enjoyed international success with millions of tons of ethylene polymers being so produced annually.
In these polymerization processes, settling legs, however, do present some problems. They represent the imposition of a “batch” or “discontinuous” technique onto a basic continuous process. Each time a settling leg reaches the stage where it “discharges” or “fires” accumulated polymer slurry it causes interferences on the pressure in the loop reactor, which is thereby not kept constant. Pressure fluctuations in the loop reactor may be larger than 1 bar. At very high monomer concentration, such pressure fluctuations may generate several problems such as the creation of gas bubbles that may cause trouble in the operation of the circulation pump. They may also provoke perturbations in the control scheme of the reactor pressure.
Various alternative product removal techniques are however known. For example, WO 01/05842 describes an apparatus for removing concentrated slurry from a flowing stream of slurry in a conduit characterized by a channel in an outlet area of the conduit, the outlet being adapted to continuously remove slurry.
EP 0891990 describes an olefin polymerization process wherein the product slurry is recovered by means of a continuous product take off, more in particular by means of an elongated hollow appendage provided on the reactor. Said hollow appendage being in direct fluid communication with a heated flash line and thus being adapted for continuous removal of product slurry.
However the above-described apparatus and processes have the disadvantage that the suspension withdrawn from the reactor still contains a large amount of diluent and of other reactants, such as the monomer, which it is then necessary to subsequently separate from the polymer particles and to treat for the purpose of reusing it in the reactor. Another disadvantage of the above-described apparatus and processes is their lack of flexibility during the phase or reaction start-up or in response to large disruptions in the normal behavior of the reactor, like sudden interruption of one of the feed streams.
It is therefore an object of the present invention to provide a polymerization process occurring in a loop reactor wherein the polymer slurry is efficiently removed from the loop reactor through sequentially operated setting legs. It is further an object of the present invention to establish non-fluctuating reaction conditions in a reactor during a polymerization process. More in particular, it is an object of the invention to preserve, pressure and to avoid pressure fluctuation in a polymerization reactor. Another object of the present invention is to increase the reactor throughput by providing stable operation conditions. A further object is to increase the monomer concentrations in the liquid medium. Another object of the present invention is to increase the weight percent (wt %) of polymer solids in the polymerization slurry circulating in the polymerization zone in the loop reactor. It is a further object of the invention to provide a flexible process that can be routinely converted to conventional setting leg removal mode in order to adapt to sudden disruption of the operating conditions caused for example by sudden large modification of the diluent or monomer feed throughput rates or start-up conditions.